The present invention relates generally to boiler economizers for maximizing heat transfer from hot products of combustion to water, and in particular to economizer bank arrangements where hot banks and cold banks are arranged next to each other so that a water coil air heater (WCAH) can be used without requiring multiple banks in series relative to the gas flow.
Economizers and air heaters perform key functions in energy generation by increasing overall boiler thermal efficiency by recovering energy from flue gas before it is exhausted to the atmosphere. Typically for each 40 F (22 C) that the flue gas is cooled by an economizer—sometimes in conjunction with an air heater—overall boiler efficiency can increase by about 1%. Economizers typically recover energy by using heat from partially-cooled flue gas to preheat feedwater before the feedwater continues on to a boiler for further heating. Water heated in an economizer can also, optionally, be routed through an air heater.
Air heaters preheat combustion air to enhance the combustion of many fuels. For example, supplying preheated air is critical for pulverized coal firing. It contributes to drying coal and to promoting stable ignition. Recycling heat into a furnace via an air heater is another a way of increasing boiler efficiency by reducing the amount of heat energy vented to the atmosphere.
In comparison to furnace water walls, superheaters, and reheaters, economizers and air heaters normally require a large amount of heat transfer surface per unit of heat transferred. This is because of the relatively small difference between the temperature of the (already significantly cooled) flue gas and the temperature of the feedwater and/or the combustion air, which receives the heat. Normally heated flue gas from a heat source, such as a furnace, first passes through superheaters and/or other heat transfer devices before reaching an economizer. By the time the flue gas reaches the economizer, it has already passed much of its original peak heat energy to other heat transfer devices, so its temperature becomes lower. The purpose of the economizer is to harvest and recycle what excess heat remains.
Economizers are primarily heat transfer surfaces used to preheat boiler feedwater before it enters, for example, a drum or a furnace surface, depending on the boiler design. Economizers typically include a number of tubes. The tubes may have fins or other structures to increase their heat absorption from gas passing over the tubes. The term “economizer” comes from early use of such heat exchangers to reduce operating costs or economize fuel usage by recovering extra energy from flue gas. Economizers also reduce the potential of thermal shock, drum level fluctuations, and water temperature fluctuations entering boiler drums or water walls.
Economizers can be used in a variety of applications, including various types of power plants and boilers, including process recovery boilers used in the paper pulp manufacturing industry. The standard practice has been to arrange long flow economizer surfaces across the full width of a boiler or other spaces where heated gas is routed.
To further improve efficiency (by increasing water to flue gas temperature differentials), heat can be removed from economizer feedwater via the addition of a WCAH in the feedwater flow path between separate cold and hot economizer banks. The WCAH improves economizer performance by removing and recycling some heat from the circulating water within the economizer process, thereby increasing the water to gas temperature differential when the water enters a successive (hotter) economizer bank. This increased temperature differential increases total heat absorption by the circulating water, and that increased heat absorption increases boiler efficiency more than the efficiency of an economizer without a WCAH unit. See FIG. 1, which shows a typical prior art arrangement of a cold bank economizer 22, a WCAH 30, and a hot bank economizer 24. In this arrangement, feedwater enters a cold bank economizer 22 at a feedwater inlet 40. While passing through cold bank economizer 22 feedwater absorbs heat energy from the flue gas flow 4 as the flue gas flows through the cold bank economizer 22. Feedwater subsequently flows through a WCAH 30, wherein a portion of the heat energy absorbed from the cold bank economizer is rejected to an air stream. The cooled feedwater subsequently absorbs additional heat energy from the flue gas flow 4 as the flue gas flows through the hot bank economizer 24. The air heated by the WCAH 30 can, for example, be used to improve fuel ignition and combustion in a furnace.
A problem with the prior art design shown in FIG. 1 is that it requires two full long flow economizer banks placed in series relative to the gas flow 4. Notice that each bank spans all or nearly all of the distance between the first side economizer wall 6 and the second side economizer wall 8 across the path of the flue gas flow 4. The first side economizer wall 6 and second side economizer wall 8 enclose the economizer banks. Thus, without at least two separate long flow economizer banks, a WCAH 30 cannot be installed in the feedwater flow path between cold and hot banks. See also FIG. 2 (showing a perspective drawing of a prior art economizer with a single continuous collection header fed by many mini-headers) and FIG. 3 (a plan view of a prior art wall-to-wall cold bank economizer).
A WCAH can theoretically be installed upstream or downstream of a single bank economizer, but will offer only nominal boiler efficiency improvement if it is not between two economizer banks in the feedwater flow path. A WCAH cannot, however, be installed at an intermediate location using a single traditional long flow (e.g. mini-header) type economizer bank. This is because the typical mini-header design feeds the mini headers 28 with continuous (inlet and outlet) collection headers 26, as shown in FIGS. 2 and 3. There is no practical location to integrate a WCAH 30 using the prior art collection headers, particularly since the WCAH 30 must be placed outside of the hot flue gas flow, typically outside the boiler wall, to function.
At the same time, it is often not practical or desirable to install two full separate economizer banks spanning the gas flow path as shown in FIG. 1. In some cases, using two separate banks in series is impractical or requires too much space, particularly when a pre-existing space is being refitted. Installing two separate full economizer banks can also add unwanted expense.
Thus, there is a need for economizer arrangements that allow the use of a water coil air heater with only a single bank economizer, with the hot and cold economizer banks in parallel, relative to the gas flow, and without the need for two economizer banks in series, relative to the gas flow.